What is VIP(Via in Pad)?
In short, the vias are drilling on the pads on the PCB can be called VIP(Via in Pad). Generally speaking, the diameter of the via in pad should not be bigger than 0.5mm, otherwise the solder paste will flow into the hole during SMT, or the flux will flow into the hole to generate gas during heating, resulting in insufficient connection strength between the device and the pad,then virtual soldering.
The VIP(via in pad) is an important part of the multilayer PCB. It is not only responsible for the main functions of the PCB, but also the cost of drilling usually accounts for 30% – 40% of the PCB manufacturing cost.
Different Categories of Via in Pad’s
From the point of view of function, the via in pad can be divided into two categories: One is for electrical connection between layers; the other is for fixing or positioning devices. In terms of process, these The VIP(via in pad) are generally divided into three categories, namely blind vias, buried vias and through vias. Blind vias are located on the top and bottom surfaces of the printed circuit board and have a certain depth. They are used to connect the surface line and the inner line. The depth of the hole usually does not exceed a certain ratio (aperture). Buried hole refers to the connection hole located in the inner layers of the printed circuit board, which does not extend to the surface of the circuit board. The above two types of holes are located in the inner layer of the circuit board, and are completed by a through-hole forming process before lamination, and several inner layers may be overlapped during the formation of the via. The third type is called a through hole, which penetrates the entire circuit board and can be used for internal interconnection or as a component mounting positioning hole. Because the through hole is easier to implement in the process and the cost is lower, most of the printed circuit boards use it instead of the other two types of via in pad.
Functions of Via Plugging
With the development of electronic products in the direction of lighter, thinner, and smaller, PCB has also been promoted to high-density and high-difficulty development, and customer requirements are getting higher and higher. The requirements for holes are getting higher and higher. For example, no solder mask ink should enter the holes, causing tin beads in the holes, no oil explosions, and difficult placement of components.
As we all know, the printed circuit board plugging procedure is a process that arises from the higher requirements to put forward by the PCB manufacturing process and surface mount technology. Its plugging functions have the following points:
- Prevent the tin from penetrating through the component surface from the via hole to cause short circuit when the PCB is wave soldered
- Avoid flux residue in the via
- Prevent the tin balls from popping up during wave soldering, causing short circuits
- Prevent surface solder paste from flowing into the hole, causing false soldering and affecting SMT
Via Plugging Experiment
Experiment one: Straight Plug Method
Drilling out the aluminum sheet that needs to be plugged. The aluminum sheet is 2 inches larger than the processing board, and the diameter of the hole is larger than the actual processing board by 0.1 mm. Make a screen or directly install it on the screen printing machine for plugging. After plugging the hole, it should not be parked for more than 30 minutes. Use 36T screen to directly screen the surface of the board. The process flow is:
- Plug hole
- Silk screen
With this process, the production cycle can be shorter, and it can ensure that the throught holes are covered soldermas very well, the plug vias are flat, and the wet film color is consistent. After HASL finished, it can ensure that the via hole is not tinned and the hole does not hide tin ball, but during via in pad plugging process, its position is easy to cause oil explosion after curing or hot air leveling, and the soldermask in the hole will be on the pad, then causing poor solderability.
Experiment Two: Grinding Method
Drill out the aluminum sheet that needs to be plugged. The aluminum sheet is 2 inches larger than the processing board, and the diameter of the hole is larger than the actual processing board by 0.1 mm. Make a screen or install it directly on a screen printing machine for plugging holes. The plugging holes must be full and protruding on both sides. It is cured and polished for surface treatment. The process flow is:
- Plug hole
- Printing solder mask
Because this process adopts plug hole curing to ensure that the via hole after HAL does not lose oil or explode, but after HAL, it is difficult to completely solve the problem of tin ball in the via hole or tin on the through hole.
Figure 1 illustrates how the VIPPO structure can influence those parasitic effects. The signal traces, which connect the BGA pads with the vias, act as inductors. Additionally, as high-speed designs typically have ground planes immediately below the outer layer, there is also a capacitive effect that is generated. With the VIPPO structure, the outer trace layer is eliminated, thereby cancelling both parasitic effects.
Figure 1: Dog-bone vs. VIPPO pad structure.
Figure 2 exhibits the VIPPO structure as compared with the VIPPO + backdrill (BD) structure. The use of backdrill with the VIPPO structure can eliminate the reflections within the unused portion of the via, which acts as a stub. The portion of the via indicated by the purple arrow is not in series with the signal path, but instead acts as a stub. Therefore, a portion of the signal is reflected back, creating an interference, which will degrade the high-speed signal performance. Hence, the purpose of the back-drill is to remove this “unused” portion of the via in order to eliminate the reflections for a cleaner signal.
Figure 2: VIPPO vs. VIPPO + backdrill structure.
With increased complexity of PCB designs for high-end networking products, the boards thicknesses are typically >120 mils and signal speeds are reaching 25 GHz and beyond. For these types of designs, backdrilling of the VIPPO structures becomes imperative.
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